Former Attorney General says Prime Minister Peter O'Neill's attempt to explain a crucial letter in a corruption case undermines his forgery claims.Kerenga Kua says Peter O'Neill's attempt to explain a crucial letter in a corruption case undermines the prime minister's later claim that the letter was forged.Mr O'Neill has confirmed to the ABC that he signed a letter in 2013 that clarified the intent of an earlier directive to process legal bills, but insists he misunderstood what he was signing.Mr O'Neill says his chief of staff, Isaac Lupari, drafted the letter for him to sign but Mr Lupari has refused to confirm that he was the author."If [Mr O'Neill] says that he didn't sign the first letter, then how is it that in the second letter he refers to the first letter by date?" former attorney-general Kerenga Kua said.Mr Kua was dismissed for questioning Mr O'Neill's leadership but remains an MP for Sinasina-Yonggamugl. A legal and political scandal still continuing after police served Mr O'Neill with an arrest warrant for official corruption on June 16.One of the key pieces of evidence is a letter dated January 24, 2012 directing the Ministry of Finance to process $28 million to Paraka Lawyers for legal work investigators say is mostly fabricated. Mr O'Neill has consistently said the letter is a forgery and did not come from his office.PNG's anti-corruption agency, Investigation Task Force Sweep, has challenged that claim, saying a Sydney-based forensic lab has confirmed the signature is that of Mr O'Neill.